1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for controlling the mixture ratio of air and fuel (air-fuel ratio: A/F) in an internal combustion engine at a desired value by supplying an appropriate quantity of fuel in accordance with the amount of intake air, and more particularly to an air-fuel ratio control apparatus which performs air-fuel ratio feedback control with an air-fuel ratio sensor (A/F sensor), capable of linearly detecting the air-fuel ratio, mounted upstream of an exhaust gas purifying catalyst.
2. Description of the Related Art
Three-way catalysts for simultaneously promoting the oxidation of unburned constituents (HC and CO) and the reduction of nitrogen oxides (NO.sub.x) in automotive exhaust have long been used on automotive engines to control exhaust emissions. For a maximum oxidation/reduction efficiency of the three-way catalyst, the air-fuel ratio (A/F), a measure of engine combustion state, must be controlled within a very narrow range (called the window) centered at stoichiometry. To achieve this, in fuel injection control in an engine, an O.sub.2 sensor (oxygen concentration sensor--see FIG. 1) is mounted that detects whether the air-fuel ratio is on the lean side or rich side with respect to stoichiometry on the basis of the concentration of residual oxygen in the exhaust gas, and air-fuel ratio feedback control is performed to correct the quantity of fuel based on the sensor output.
In such an air-fuel ratio feedback control configuration, the O.sub.2 sensor for detecting the concentration of oxygen is mounted as close as possible to the combustion chamber, that is, on the upstream side of the catalytic converter. Furthermore, to compensate for variations in the output characteristic of the O.sub.2 sensor, a double O.sub.2 sensor system having a second O.sub.2 sensor on the downstream side of the catalytic converter has also been introduced for commercial use. The principle of this system is based on the fact that, on the downstream side of a catalytic converter, the exhaust gas is thoroughly stirred, and its oxygen concentration is almost in equilibrium by the action of the three-way catalyst; consequently, the output of the downstream O.sub.2 sensor changes mildly compared with the upstream O.sub.2 sensor, and thus indicates whether the air-fuel mixture as a whole is on the rich side or lean side. In the double O.sub.2 sensor system, sub air-fuel ratio feedback control is performed using the O.sub.2 sensor downstream of the catalyst in addition to the main air-fuel ratio feedback control by the O.sub.2 sensor upstream of the catalyst, and the air-fuel ratio correction factor used in the main air-fuel ratio feedback control is corrected based on the output of the downstream O.sub.2 sensor, to accommodate variations in the output characteristic of the upstream O.sub.2 sensor and thereby improve the precision of the air-fuel ratio control.
Even when such a precise air-fuel ratio control is performed, if the catalyst deteriorates due to exposure to exhaust gas heat or poisoning by lead and other contaminants, a satisfactory exhaust gas purification performance cannot be obtained. To address this problem, a variety of catalyst deterioration detection devices have been proposed in the prior art. One such device diagnoses the deterioration of the catalyst by detecting a decrease in an O.sub.2 storage effect (the function to store excessive oxygen and reuse it for the purification of unburned exhaust emissions) after warmup using an O.sub.2 sensor mounted on the downstream side of the catalyst. That is, deterioration of the catalyst leads to a degradation in the purification performance after warmup, and the above device deduces the degradation of the purification performance from a decrease in the O.sub.2 storage effect; more specifically, by using an output signal from the downstream O.sub.2 sensor, the device obtains response curve length, feedback frequency, etc. and detects the decrease of the O.sub.2 storage effect and, hence, the deterioration of the catalyst. For example, Japanese Patent Unexamined Publication No. 5-98948 (corresponding U.S. Pat. No. 5,301,501) discloses a device which obtains the output response curve length of the downstream O.sub.2 sensor during feedback control toward stoichiometry, and based on that, detects catalyst deterioration.
On the other hand, recent years have also seen the development of an internal combustion engine in which the air-fuel ratio is controlled so that the three-way catalyst can consistently provide a constant and stable purification performance. That is, the O.sub.2 storage capability is such that, when the exhaust gas is in a lean state, excessive oxygen is adsorbed, and when the exhaust gas is in a rich state, the necessary oxygen is released, thereby purifying the exhaust gas; however, such a capability is limited. To make an effective use of the O.sub.2 storage capability, therefore, it is important to maintain the amount of oxygen stored in the catalyst at a prescribed level (for example, one-half the maximum oxygen storage amount) so that the next change in the air-fuel ratio of the exhaust gas can be accommodated, whether it is a change to a rich state or a lean state. When the amount of oxygen is maintained in this manner, a consistent O.sub.2 adsorption/desorption function can be achieved, thus ensuring a consistent oxidation/reduction performance of the catalyst.
In the internal combustion engine in which the O.sub.2 storage amount is controlled to a constant level to maintain the purification performance of the catalyst, an air-fuel ratio (A/F) sensor (see FIG. 2) capable of linearly detecting air-fuel ratio is used, and feedback control (F/B control) is performed based on a proportional-integral operation (PI operation). That is, a feedback fuel correction amount is calculated by
Next fuel correction amount=K.sub.p *(Present fuel error)+K.sub.s *.SIGMA.(previous fuel errors) PA1 Fuel error=(Fuel amount actually burned in cylinder)-(Target fuel amount in cylinder with intake air at stoichiometry) PA1 Fuel amount actually burned in cylinder=Detected value of air amount/Detected value of air-fuel ratio PA1 K.sub.p =Proportional gain PA1 K.sub.s =Integral gain
where
As can be seen from the above equation for the fuel correction amount, the proportional term is the component that acts to maintain the air-fuel ratio at stoichiometry, as in the feedback control using an O.sub.2 sensor, while the integral term is the component that acts to eliminate the steady-state error (offset). That is, by the action of the integral term, the O.sub.2 storage amount in the catalyst is maintained at a constant level. For example, when a lean gas occurs as a result of abrupt acceleration or the like, the air-fuel ratio is enriched by the action of the integral term, offsetting the effect of the lean gas.
As described above, in the air-fuel ratio feedback control based on the output voltage of the A/F sensor, control is performed in such a manner as to increase the fuel correction amount as the difference between the output voltage and the target voltage (voltage equivalent to stoichiometry) increases; accordingly, if there occurs a deviation in the output characteristic (out of range) or response characteristic of the A/F sensor, it becomes difficult to achieve a desired air-fuel ratio feedback control. For example, as shown in FIG. 3A, if the A/F sensor shows an excessive response, and the fluctuation of the output voltage VAF (indicated by a solid line) of the A/F sensor becomes greater than the fluctuation of the voltage (indicated by a dashed line) that the sensor should indicate in response to the real A/F (the voltage equivalent to real A/F), the fuel correction amount will become larger than originally designed and the time required to return to the stoichiometry equivalent voltage (target voltage), that is, the cycle of the air-fuel ratio fluctuation, will become shorter. Conversely, as shown in FIG. 3B, if the fluctuation of the output voltage VAF (indicated by a solid line) of the A/F sensor becomes smaller than that of the real A/F equivalent voltage (indicated by a dashed line) because of a deterioration of the A/F sensor response, the fuel correction amount will become smaller than originally designed and the cycle of the air-fuel ratio fluctuation will become longer. Accordingly, in air-fuel ratio feedback control using an A/F sensor, it is important that a deviation in the characteristic of the A/F sensor be compensated for to achieve an air-fuel ratio control accuracy of an acceptable level. Furthermore, it is desirable that a notification be made when a deviation or deterioration is detected in the characteristic of the A/F sensor.
In internal combustion engines in which air-fuel ratio feedback control is performed based on the output voltage of an A/F sensor, an O.sub.2 sensor may also be provided on the downstream side of the catalyst to perform sub air-fuel ratio feedback control. In the sub air-fuel ratio feedback control, displacements of the output voltage VOS of the O.sub.2 sensor from the target voltage VOST are summed and, based on the sum value, the output voltage VAF of the A/F sensor is corrected, thereby bringing VOS close to the target voltage VOST (that is, the center of the air-fuel ratio fluctuation is gradually shifted until the target voltage is reached). In this case also, catalyst deterioration can be detected by detecting a decrease in the catalyst's O.sub.2 storage effect using the O.sub.2 sensor, as in the double O.sub.2 sensor system.
More specifically, when the O.sub.2 storage performance drops due to deterioration of the catalyst, the output voltage VOS of the O.sub.2 sensor mounted downstream of the catalyst changes in a short cycle; on the other hand, when the catalyst is normally functioning, the output voltage VOS changes only slightly in a longer cycle because of the catalyst's O.sub.2 storage effect. In detecting catalyst deterioration based on the response curve length of the O.sub.2 sensor output voltage VOS, the response curve length LVOS of the VOS is obtained over a predetermined monitoring period, and when the value exceeds a critical value, the catalyst is judged as being deteriorated. Here, the critical value is determined according to the response curve length LVAF of the A/F sensor output voltage VAF calculated over the same predetermined period, and is made larger as LVAF becomes longer. For example, in FIG. 4, when the point expressed by (LVAF, LVOS) is above the critical value curve shown in the figure, the catalyst is judged as being deteriorated.
However, when the fluctuation of the A/F sensor output voltage VAF (indicated by the solid line) shows a larger value than the actual A/F fluctuation (indicated by the dashed line) because of an excessive response of the A/F sensor, as shown in FIG. 3A, the resulting value will be displaced and indicate point 1! in the normal region in FIG. 4 despite the condition that should indicate point (1) in the abnormal region. In that case, the catalyst is judged as being normal when it should be judged as being abnormal. Conversely, when the fluctuation of the A/F sensor output voltage VAF (indicated by the solid line) shows a smaller value than the actual A/F fluctuation (indicated by the dashed line) because of deterioration of the A/F sensor response, as shown in FIG. 3B, the resulting value will be displaced and indicate point 2! in the abnormal region in FIG. 4 despite the condition that should indicate point (2) in the normal region. In that case, the catalyst is detected as being abnormal when it should be judged as being normal. Therefore, when judging catalyst deterioration also, it is important to compensate for a deviation in the characteristic of the A/F sensor if the accuracy of catalyst deterioration judgement is to be improved.